A conventional internal combustion engine comprises a set of cylinders arranged in line, a piston reciprocable in each cylinder and connected to a crankshaft, each piston being either in phase or out of phase with the others by a phase angle A.degree. or an integral multiple thereof, a plurality of rotatable cams for actuating inlet and exhaust valves of each cylinder, and a cam drive mechanism for rotating the cams in a predetermined phase relationship with the crankshaft to open each valve in sequence through a desired angle of rotation of the crankshaft. In a conventional 4-stroke engine, the cam drive mechanism rotates the cams once for every two rotations of the crankshaft.
Such drive mechanism suffer from the disadvantage that the periods (i.e., angles of rotation of the crankshaft) for which the valves are opened during each cycle of the engine are fixed. In practice, the optimum periods vary with the operating conditions of the engine. For example, when the engine is operating at high speeds, maximum power would be achieved by opening the inlet and exhaust valves for relatively longer periods within each cycle, whereas at low engine speeds and low loads, shorter operating periods improve the fuel efficiency of the engine. An improvement of fuel efficiency at low speeds could also be obtained by altering the operation of the exhaust and inlet valves to reduce the period for which both valves are open together.
British Patent Specification No. 1522405 discloses a cam drive mechanism that includes means for varying the angle of rotation of the camshaft through which the valves are opened to suit varying engine operating conditions. This is achieved by combining the rotational movement of the cams with oscillations about their axis of rotation which also have a predetermined phase relationship with the crankshaft and varying the amplitude of these oscillations to match the change in the period for which the valves are opened to the engine conditions.
The drive mechanism described in British Patent Specification No. 1522405 comprises an intermediate drive shaft driven at half the speed of the crankshaft and connected to the camshaft by an eccentric coupling. Displacement of the axis of rotation of the intermediate drive shaft radially with respect to the axis of the camshaft produces a combined rotational and oscillatory movement in the camshaft, the frequency of the oscillatory movement being equal to the frequency of rotation of the camshaft. However, in the construction described in that specification, the required phases of these oscillations differ for each cam and, therefore, an individual eccentric coupling driving an individual camshaft is required for each cylinder. Hence, the drive mechanism is relatively complicated and expensive to produce in a multi-cylinder engine.